In terms of "splitting" it feels like its more likely that certain parts of personality were "walled off" rather than some parts splitting away from a larger part. Over time maybe the walled off parts develop more distinct characteristics the more they are needed to act. So some became better developed than others simply because they have gotten more air time. That's how I think of it in us anyway, and it seems to play out. Our alters who are more highly developed all have a greater role to play in life. Our more fragmented alters are very limited in what they can do, because they've only dealt with one or two very limited things. Those ones seem to be ones who handled just a tiny fragment of an abuse episode. They don't come out at all apart from if they are reliving their trauma in flashback.